The picture above shows a rocky outcrop which a team of NASA scientists has named "Hottah", after Hottah Lake in Canada's Northwest Territories. It doesn't seem much, but what you're looking at is exposed bedrock, the kind that you find at the bottom of a river.

The bedrock itself is made up of smaller fragments of rock cemented together—what geologists call a sedimentary conglomerate. In the past, it would have looked flat but the scientists think it must have been disrupted, maybe by meteorites, to give it a slant. The site lies between the north rim of Gale Crater and the base of Mount Sharp.

But the main evidence suggesting that this is a riverbed is... gravel. NASA scientists have been studying images of the stones cemented into the rock, and their size and shape points to the fact that this used to be a fast-flowing stream. The pieces of gravel are particularly round, which suggests that they've been transported long distances by water; at up to a couple of inches in size, they're definitely too big to have been moved by wind. William Dietrich of the University of California, Berkeley, explains:

"From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep... This is the first time we're actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it."

Amazing: a hip-deep river, right there on Mars. The next step is for Curiosity to measure the elemental composition of the materials to try and get a better understanding of what the environment used to be like. Of course, where there's water, there could've been life. Mars Science Laboratory Project Scientist John Grotzinger explains:

"A long-flowing stream can be a habitable environment. It is not our top choice as an environment for preservation of organics, though. We're still going to Mount Sharp, but this is insurance that we have already found our first potentially habitable environment."

While there's still plenty for Curiosity to do, this new finding confirms that streams and rivers once flowed on Mars. The implications that has for the one-time existence of life on Mars are huge. [NASA]